The present invention relates to a fluid dynamic bearing wherein a lubricant filled in the clearance between a shaft and a sleeve for rotatably supporting the shaft is used as a pressure generation liquid. The fluid dynamic bearing in accordance with the present invention is used, for example, for magnetic disk drive spindle motors in magnetic disk apparatuses, polygon-mirror rotation drive apparatuses in high-speed digital copiers, laser printers, and rotary magnetic head apparatuses in video tape recorders, etc. More particularly, the present invention relates to a fluid dynamic bearing having means for preventing the charging of static electricity in the rotation portion thereof during high-speed rotation, and a magnetic disk apparatus.
The fluid dynamic bearing comprises at least a shaft, a sleeve for rotatably supporting this shaft and a lubricant serving as a lubrication fluid filled in the clearance between the shaft and the sleeve. In the fluid dynamic bearing, dynamic pressure generation grooves for raising the pressure of the filled fluid during rotation are formed on at least one of the outer circumferential face of the shaft and the inner circumferential face of the sleeve. Since the lubricant is filled in the clearance at the rotation portion on which the dynamic pressure generation grooves are formed as described above, the dynamic pressure of the lubricant is raised by the pumping action of the dynamic pressure generation grooves during the rotation of the shaft or the sleeve, and the shaft is held in a noncontact state by the sleeve via the lubricant.
In the driving state of the fluid dynamic bearing, one of the shaft and the sleeve rotates at high speed in a state of not making contact with each other via the lubricant. As a result, the rotation portion is electrically charged owing to the flow of the lubricant. In addition, in the case when the fluid dynamic bearing configured as described above is used for a magnetic disk apparatus, magnetic disks cause friction with air owing to the rotation of the magnetic disks and is electrically charged. This amount of the charge becomes larger as the speed of the magnetic disks is higher. The magnetic disks are secured to the sleeve, and the sleeve is rotatably held on the shaft in a noncontact state via the nonconductive lubricant. Hence, the charge generated on the magnetic disks serving as the rotation portion has no outflow passage, whereby the charge builds up gradually on the rotation portion, such as the magnetic disks and the sleeve. The static electricity charged as described above is in danger of being discharged suddenly during the operation of the magnetic disk apparatus. In the case when this kind of undesirable discharge occurred suddenly, there was a danger of causing malfunctions, such as read errors or write errors, in the magnetic disk apparatus and of causing static damage to the magnetic disks or the like. Hence, the magnetic disk apparatus using the conventional fluid dynamic bearing was unstable in operation and had problems in reliability.
To solve these problems, in the conventional fluid dynamic bearing, an electrical conductivity imparting agent, such as conductive polymer, carbon black or alkyl sulfonate, was added to the lubricant so that the charge is released via the lubricant.
However, in the case when the electrical conductivity imparting agent was added to the lubricant as described above, the viscosity of the lubricant increased, thereby increasing torque loss during high-speed rotation and causing the degradation of the lubricant owing to heat generation due to the torque loss.